In for example an FT-IR spectrometer, infrared or near infrared radiation is directed from a source of such radiation towards a sample under investigation. Radiation transmitted by or reflected from the sample is received at a receiver and the output of the receiver is processed by a signal processor in order to obtain the spectral characteristics of the sample. In carrying out measurements it is first necessary to obtain background measurements, that is to say to measure the background spectrum without a sample in place at the sample station. Subsequently, measurements are made with the sample in place and the desired sample spectrum is obtained from the ratio of the measurement obtained with the sample in place to the background measurement. Conventionally background measurements are made at times which are very close to the time of the sample measurement in order to avoid any effects from instrument drift. The frequency of new background measurements depends on the stability of the spectrometer and the accuracy required. Also, because there is noise associated with the background measurement equal measurement times are usually used for both the sample and background measurements. As will be known to those skilled in the art during such measurement times it is the practice to carry out multiple scans which are then averaged in order to reduce the effects of noise. This procedure adds to the time necessary in order to obtain a sample spectrum.